"The IAU notes the very rapid pace of discovery of bodies within the Solar system over the last decade and so our understanding of the Trans-Neptunian Region is therefore still evolving very rapidly. This is in serious contrast to the situation when Pluto was discovered. As a consequence, The IAU has established a Working Group to consider the definition of a minimum size for a Planet. Until the report of this Working Group is received, all objects discovered at a distance from the Sun greater than 40 AU will continue to be regarded as part of the Trans-Neptunian population."

UPDATE August 24, 2006

RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three
distinct categories in the following way:

(1) A planet1 is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a
hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass
for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape2,

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite.

(3) All other objects3 orbiting the Sun shall be referred to collectively as
"Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.2An IAU process will be established to assign borderline objects into either
dwarf planet and other categories.3These currently include most of the Solar System asteroids, most Trans-Neptunian
Objects (TNOs), comets, and other small bodies.

Mercury is always very near the Sun
and difficult to see in the early sunset or pre-dawn.
It is best pursued with small telescopes and binoculars when at its greatest
eastern or western elongation, that is when its
angular deviations are at a maximum with the Sun as it appears to swing back and forth
from the morning to the evening skies.

Venus shows phases when viewed with a telescope from the perspective of Earth
because it is closer to the Sun than the Earth. When seeing part of its unlit side
it presents a crescent or gibbous shape, much like our Moon. The reflected sunlight
from Venus is very bright. On a dark moon-less night you may see your shadow from
the light of Venus.

It was not until the 16th century that the idea began to spread and be accepted that
the Earth was a planet in the same sense as the other planets that orbit the Sun.

Earth has only one moon, which is unlike any of the other satellite
bodies in the solar system. It is extremely large yet relatively distant from its parent planet.
Other moons of the solar system tend to be classified as natural satellites or captured planetoids. The naturals
are mostly large with circular orbits, very near the parent planet, aligned with the planet's equator. Captured
planetoids tend to be small with eccentric orbits inclined to the equator and
relatively distant from the parent planet. The Earth's moon does not fit into either
of these categories very well.

Thousands of artificial satellites have been placed in orbit around the Earth.

Viewing the surface features depends greatly on seeing conditions.
Usually a 6" or 8" diameter telescope with reveal the Great Red Spot, which is
a giant cyclonic storm about three times the size of Earth. The lateral bands
on the surface are alternately colored: The darker ones ranging from brick red to light peach and
the lighter ones ranging from aqua to light grey. Sometime you will see a black dot slowly travelling
across the surface of the planet; a shadow from an intervening moon.

Jupiter has more than 60 known moons including four that were discovered in 1610
when Galileo was the first to view it through a telescope. Their names are
Io, Europa, Ganymede and Callisto. They can be seen easily through binoculars
or a small telescope.

Saturn's rings are beautiful even in small amateur telescopes, especially
when the planet is at its greatest tilt to the Earth, exposing the maximum
surface area of the ring system.
The rings are extremely thin compared to the width. They are about 250,000
km or so in diameter, but they're less than one kilometer thick.
The ring particles are composed primarily of water ice,
with some rocky particles with icy coatings.

Saturn has more than 30 moons. The largest, Titan, is prominent in views
through a small telescope.

It is difficult for beginners with small telescopes to find Uranus. It
may appear as a bluish-green star. In slightly larger scopes it will
look more like a disk and less like a point.

An interesting thing about this planet is that unlike most of the planets,
which spin on an axis nearly perpendicular to the plane of the ecliptic,
Uranus' axis is almost parallel to the ecliptic. It seems tipped on its side.

Uranus has at least 27 moons. The largest are named after characters
in Shakespeare plays. Others are named after characters in writings
by Alexander Pope.

Like Uranus it is difficult for beginners with small telescopes to find it.
It usually appears simply as a bluish-green star. In slightly larger scopes it will
look more like a disk and less like a point of light.

Now officially classified as a "dwarf planet," Pluto crosses inside the orbit of Neptune
for part of its orbit. It is smaller by far than the planets.
Pluto is smaller than seven of the solar system's moons.
mass: 1.27 x 1022 kg
orbit: 5,913,520,000 km (39.5 AU) from Sun (average)
diameter: 2274 km (at the equator)

In earth-bound telescopes Pluto looks like a star. To know if you are observing it,
make a drawing of the star field when you think you have found Pluto. Come back
in a couple of weeks and find the field again. If it was Pluto it will have moved
slightly in relation to the backdrop of stars.

Pluto has a moon named Charon which is named after the mythical ferry boat
operator who take souls across the River Acheron to Hades.